Cells 2015, 4, 452-473; doi:10.3390/cells4030452 OPEN ACCESS cells ISSN 2073-4409 www.mdpi.com/journal/cells Review Multiple Export Mechanisms for mRNAs Mildred Delaleau and Katherine L. B. Borden * Department of Pathology and Cell Biology, Institute for Research in Immunology and Cancer, Université de Montréal, Montréal, QC, H3C 3J7, Canada; E-Mail: [email protected] * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +1-514-343-6291; Fax: +1-514-343-5839. Academic Editor: Birthe Fahrenkrog Received: 31 July 2015 / Accepted: 21 August 2015 / Published: 28 August 2015 Abstract: Nuclear mRNA export plays an important role in gene expression. We describe the mechanisms of mRNA export including the importance of mRNP assembly, docking with the nuclear basket of the nuclear pore complex (NPC), transit through the central channel of the NPC and cytoplasmic release. We describe multiple mechanisms of mRNA export including NXF1 and CRM1 mediated pathways. Selective groups of mRNAs can be preferentially transported in order to respond to cellular stimuli. RNAs can be selected based on the presence of specific cis-acting RNA elements and binding of specific adaptor proteins. The role that dysregulation of this process plays in human disease is also discussed. Keywords: nuclear mRNA export; selective export; NXF1 pathway; CRM1 pathway 1. Introduction Accurate and dynamic control of gene expression is critical for all cells in order to respond to cellular stresses, environmental stimuli and to properly regulate proliferation and growth. Gene expression can be controlled not only at the levels of transcription and translation, but also by controlling the nuclear-cytoplasmic export of mRNAs that encode proteins involved in these cellular processes. Specific subsets of mRNAs can be differentially exported leading to alterations in the proteome by controlling the levels of transcripts available to the translation machinery. This specificity is often underpinned by the presence of specific sequence elements within the untranslated regions Cells 2015, 4 453 (UTRs) of these transcripts. These elements act as USER codes allowing groups of transcripts acting in similar activities to be coordinately exported as in the RNA regulon model [1,2]. To be efficiently exported transcripts must undergo several maturation steps including capping, splicing and 3′ end formation. Then transcripts must traverse the nuclear envelope generally via the nuclear pore complex. This involves docking onto the nuclear basket, transiting the central channel of the nuclear pore and being released from the cytoplasmic fibrils (Figure 1) [3–7]. mRNAs bound to particular protein co-factors associate with specific transporters to traverse the nuclear pore complex (NPC). Once released into the cytoplasm, mRNAs can then become available to the translation machinery. All of these steps are highly regulated and show differential specificity and dynamic response to a wide variety of stimuli. Nuclear mRNA export was once considered to be a constitutive housekeeping activity but now is seen as a highly regulated process that can be dysregulated in and contribute to human disease. In this review, we highlight some of the recent advances in our understanding of mRNA export in mammals and discuss their implications for human disease. Figure 1. Schematic cross-section of the nuclear pore complex showing a subset of Nups discussed in this review as well as the major features of nuclear pore complex (NPC). INM indicated inner nuclear membrane and ONM, outer nuclear membrane. 2. General Features of Bulk mRNA Export Transcripts in the nucleus are transcribed as pre-mRNA and after multiple processing steps [8], these now mature mRNAs are transported to the cytoplasm to allow their translation into proteins (Figure 2) [6,7,9–12]. Recruitment of export factors to nascent mRNA start co-transcriptionally [13,14] and correctly processed mRNAs are targeted for export in the form of large ribonucleoprotein complexes called mRNPs [15]. The processing of pre-mRNA to mature mRNA includes three key steps: capping, splicing and 3′ end processing. The mature mRNA is then targeted to and translocated through the nuclear pore complex (NPC) to reach the cytoplasm and can then be translated [3,6,9,16,17]. The most common transporters for mRNA export are the protein NXF1 (also known as TAP) [18–20] and Cells 2015, 4 454 CRM1 [3–5,21]. Both NXF1 and CRM1 utilize adaptor proteins to increase the affinity of NXF1 for its RNA targets [22] or in the case of CRM1, to associate with the target RNA [4,5,23]. Figure 2. Global overview of processing and export. The different processing steps include methyl 7-guanosine capping (1); splicing (2) and the 3′ end processing (3); The mRNA is then considered mature and can be exported. NXF1 is the transporter for bulk mRNA and works in association with p15 (Nxt1) (4); Next, the export mRNP, reaches the nuclear pore complex (5); where NXF1 interacts with TPR (Translocated promoter region) to dock mRNP to the nuclear basket (6); The interaction with TPR will initiate the translocation through the nuclear channel (7); Interaction with the cytoplasmic fibrils of the NPC (composed of RANBP2) and key co-factors (Gle1 and DDX19) will release the mRNA into the cytoplasm (8). The typical means to transverse the nuclear envelope is through the nuclear pore complex (NPC). NPCs permit the passage of proteins, RNAs and solutes, between the nucleus and the cytoplasm. NPCs are 60–125 MDa structures composed of approximately 30 different proteins called nucleoporins (Nups) [24–26]. The Nups form an octagonal structure containing a central cylindrical channel, cytoplasmic fibrils, and a nuclear basket. The Nups can be divided into 3 different groups: membrane Nups that anchor NPCs to the nuclear envelope, structural Nups (also called scaffolding Nups) and the FG Nups which contain repetitive sequence of phenylalanine and glycine called FG repeats and typically play a role in the transit of cargoes through the central channel [24–30]. While nucleocytoplasmic transport of small molecules (less than about 40 kDa) occurs via diffusion [31]; larger complexes including mRNA export RNPs need energy to facilitate directionality [32–35]. For bulk mRNA, this is achieved through an ATP helicase cycle involving Gle1, DDX19 and inositol hexakisphosphate (IP6) [32,34,36–39]. Once export RNPs arrive at the nuclear pore, export can be generally considered as three distinct steps: docking onto the nuclear basket, transit through the central channel and release of mRNA cargoes at the cytoplasmic fibrils [3,6,7]. Cells 2015, 4 455 3. NXF1 Mediates Export for Bulk mRNA Export NXF1 mediates the nuclear export of bulk mRNAs [3,6,9,16]. NXF1 associates with key factors including REF/ALY and proteins of the THO complex in order to directly bind target mRNAs with high affinity [19,22,40]. Association of the mature mRNP with NXF1 allows docking with the nuclear basket nucleoporins including TPR, Nup153, Nup98 and Rae1, and subsequent translocation through the central channel [41–43]. The critical role for NXF1 in the pathway is highlighted by the observation that NXF1 is essential for cell viability and NXF1 knockdown leads to accumulation of bulk poly(A) mRNA in the nucleus [19,44–47]. NXF1 was first identified as an export factor in Mason Pfizer monkey virus RNA containing the constitutive transport element (CTE) [48–50], and later studies showed it was important for bulk cellular mRNA export [47,50]. NXF1 is composed of multiple domains [6,18,22,42,51–55] including: the RNA recognition motif (RRM) that permits NXF1 to bind non-specific RNA with a low affinity but with increased affinity in the presence of adaptors such as ALY/REF; and a leucine rich repeat domain (LRR) in conjunction with the RRM domains can bind the CTE element. The remaining two domains of NXF1 permit the translocation through the central channel by interacting with the FG Nups. For NXF1 mediated bulk mRNA export, the most common adaptor is ALY/REF [22,40]. In order to associate with ALY/REF, NXF1 must associate with the transcription export complex (TREX) [56–59]. Capping and splicing events permit the recruitment of the TREX to the 5′ end of the mRNA [60]. The TREX complex is composed of the THO complex (Thoc1/hHpr1, Thoc2, Thoc3/hTEX1, Thoc5/FMIP, Thoc6 and Thoc7) [61] and the proteins UAP56, ALY/REF and CIP29. The THO complex assembles in an ATP dependent manner with the RNA helicase UAP56, ALY/REF and CIP29 [62,63]. The ability of these factors to associate with the export RNP is reliant on correct mRNA processing thereby providing a link between mRNA processing and export [64]. For instance, UAP56 is also a component of the exon junction complex (EJC) [65], which is deposited on the mRNA as a marker of completed splicing (see below). Another example is that the THO complex with ALY/REF interacts with the nuclear Cap Binding Complex (CBC) [66], which binds the m7G cap on the 5′ end of the mRNA. Thus, capping is also linked to more efficient mRNA export. The THO complex was shown to interact with mRNA during 3′ end formation, and mutation of THO leads to nuclear accumulation of RNP containing nuclear pore components and polyadenylation factors [67]. Thus, generally, TREX recruits NXF1/p15 to cargo mRNAs once they have been fully processed thereby favouring the export of mature transcripts. After processing of mRNAs, an export-competent NXF1-mRNP is formed in the nucleoplasm which needs to reach the nuclear periphery in order to dock onto the nuclear basket and be transported. One possibility is that another complex, TREX-2, transports the mRNP from the place of transcription and mRNA processing to the nuclear basket [68,69]. In humans, TREX-2 is comprised of GANP, ENY2, CETN2/CETN3, PCID2 and DSS1.